Variable spectral width optical noise source

ABSTRACT

An optical noise source. An optical amplifier produces unpolarized optical noise by spontaneous emission. The optical noise is emitted from one side of the amplifier and filtered by a bandpass filter to attenuate any noise having a wavelength outside a desired bandspread. The filtered noise is reflected back through the amplifier for one additional amplification and then emitted from the other side of the amplifier through a nonreflecting output.

This is a divisional of copending application Ser. No. 07/860,636 filedon Mar. 30, 1992, now U.S. Pat. No. 5,272,560.

BACKGROUND OF THE INVENTION

This invention relates to a system and apparatus for generating anoptical noise in a predetermined bandwidth. This optical noise can beused in many applications, such as photodetector calibration and whitelight spectroscopy. In photodetector calibration the optical noiseoutput, which is relatively flat over a certain bandwidth, is sent to aphotodetector. The photodetector's electrical response is then examinedon a spectrum analyzer to find distortions which may be caused by thefrequency response of the photodetector. In white light spectroscopy,the optical noise output is sent to the material being tested and theabsorption spectrum is analyzed.

Prior art optical noise generators include two-pass noise generatorswhich use amplified spontaneous emission (ASE). "High Power Compact 1.48μm Diode Pumped Broadband Superfluorescent Fibre Source at 1.58 μm"; H.Fevrier, et al.; Electronic Letters, Vol. 27, No. 3; Jan. 31, 1991;gives an example of such a noise generator. This article discloses theuse of the optical amplifier in a two-pass noise generator as shown inFIG. 2. The optical amplifier 10 may consist of a doped amplifying fiber16 used as the gain medium and a pumping laser 12 which sends opticalenergy to the doped amplifying fiber via a wavelength divisionmultiplexer (WDM) 14.

Optical noise is spontaneously emitted in the doped amplifying fiber 16powered by the pumping laser 12. In the system disclosed by Fevrier,optical noise created by spontaneous emission travels through the dopedamplifying fiber and is amplified. The amplified optical noisecomponents then go to a mirror 18 which reflects the amplified opticalnoise components back to the optical amplifier. The optical amplifieramplifies the optical noise components a second time, and then thetwice-amplified optical noise components travel to the output.

The pumping frequency of the pumping laser 12 is chosen so that thefrequency is absorbed by the doped amplifying fiber 16. The energy fromthe pumping laser 12 goes through the wavelength division multiplexer 14to pump the doped amplifying fiber 16 to a higher energy state, so thatthe doped amplifying fiber 16 will amplify optical signals such asoptical noise components coming in through the optical path, and so thatthe doped amplifying fiber 16 will spontaneously emit light energy.

Looking at FIG. 2, the wavelength division multiplexer 14 works bymultiplexing the pumping frequency on line B onto line C, so that thedoped amplifying fiber 16 can absorb the pumping frequency and amplifythe optical signal on the path. Signals going into the wavelengthdivision multiplexer (WDM) 14 from line C will be de-multiplexed intotwo signals: on line A, the signal which contains the optical noisecomponents not within the pumping frequency; and on line B, the opticalsignals of the pumping frequency are sent back to the pumping laser.

Other similar prior art systems use a filter at the output of the noisegenerator so that the optical noise components will be within a desiredpredetermined bandwidth. Because the filter is placed at the output ofthe optical path, the optical amplifier amplifies optical noisecomponents that are not within the predetermined bandwidth, during thesecond amplification of the optical noise components. This unnecessaryamplification of optical noise components outside the predeterminedbandwidth may cause the optical amplifier to saturate. If the amplifiersaturates, the optical noise components within the predeterminedbandwidth are not amplified as much as the components would be amplifiedif the optical amplifier were unsaturated. Additionally, amplifying theoptical noise components outside the predetermined bandwidth expendspump power from the pumping laser 12.

It is therefore an object of the present invention to provide a noisesource that efficiently uses pump power.

A further object of the invention is to have a noise source thatconcentrates the available noise power in a narrow optical bandwidth.

SUMMARY OF THE INVENTION

An advantage of the present invention is the placement of a filter sothat the optical amplifier as a noise source does not amplify opticalnoise components outside the bandwidth of interest during at least oneamplification. This placement of the filter may prevent the opticalamplifier from becoming saturated by noise outside the filter bandwidth.In accordance with the principles of the present invention, the aboveand other objectives are realized by using an apparatus for creating anoptical noise of a predetermined bandwidth. The apparatus includes anamplifying means in an optical path for amplifying optical noisecomponents. This amplifying means produces unpolarized optical noise byspontaneous emission.

Furthermore, the apparatus includes a reflecting means in the opticalpath for reflecting the optical noise produced by the amplifying meansback to the amplifying means for at least one additional amplification.The amplifying means also includes a filter means for filtering out theoptical noise components outside the predetermined optical bandwidth andfor passing optical noise components within the predetermined opticalbandwidth. The filter means is located in the optical path so thatoptical noise components passed by the filter means are amplified by theamplifying means during an additional amplification.

Additionally, the above and other objectives are realized by using amethod for creating optical noise of a predetermined bandwidth. Themethod comprises the steps of creating unpolarized optical noise byspontaneous emission, thereafter amplifying the optical noise in anamplifying means, reflecting the amplified optical noise components totravel towards the amplifying means, filtering out the optical noisecomponents outside the predetermined bandwidth and passing optical noisecomponents within the predetermined bandwidth, and thereafter amplifyingthe filtered optical noise components that are passed in the filteringstep.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of the optical noise source apparatus of thepresent invention;

FIG. 2 is a schematic view of the prior art optical amplifier includinga pumping laser, a wave-length division multiplexer, and a dopedamplifying fiber;

FIG. 3 shows the four-pass noise source of the present invention;

FIG. 4 shows a schematic view of an alternate four-pass noise sourcedesign of the present invention; and

FIG. 5 is a schematic view of a four-pass noise source of the presentinvention where the paths of the optical noise components are shownbelow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic view of the two-pass noise source of the presentinvention. An optical amplifier 2 such as the prior art opticalamplifier shown in FIG. 2 is placed in the optical path. The componentsof the optical amplifier in the preferred embodiment include the pumpinglaser, WDM and doped amplifying fiber as described in the discussion ofFIG. 3 below.

It is to be understood that other types of optical amplifiers could beused to create and amplify optical noise. For example, a co-propagatingoptical amplifier could be used. The optical amplifier shown in FIG. 2is called a counter-propagating optical amplifier since the pumpingsignal from the pumping laser travels to the amplifying fiber in theopposite direction from the optical noise as it leaves the noisegenerator. A co-propagating optical amplifier would have the WDM andpumping laser located to the left of the doped amplifying fiber so thatthe pumping signal travels in the same direction as the optical noise asthe optical noise leaves the noise generator.

The mirror 6 is used to reflect the optical noise back to the amplifierand can alternately be replaced by a Sagnac loop, which is a 3 dBcoupler connected to a loop of fiber. In general, any means that causesthe optical noise components to travel back to the amplifying means maybe used as a reflective means and is within the scope of the invention.A filter 4 is also placed in the optical path. One embodiment uses atransmissive tunable filter which is tunable between 1515-1560 nm with a1 nm to 5 nm bandwidth. The noise source with a tunable filter mayoperate as a tunable non-coherent optical source.

In the two-pass noise source of FIG. 1, unpolarized optical noise iscreated by spontaneous emission in the optical amplifier 2. Morespecifically, the spontaneous emission occurs in the doped amplifyingfiber 16 of the prior art optical amplifier shown in FIG. 2. Theunpolarized optical noise can either go towards the mirror 6 or towardsthe output 8. If the unpolarized optical noise goes towards the mirror6, the optical noise is amplified in the optical amplifier a first time,and is filtered in the filter 4. The filter 4 filters the once-amplifiedoptical noise components to within the predetermined bandwidth, andpasses these filtered components to the mirror 6. The mirror 6 reflectsthe amplified optical noise components back through the filter towardsthe optical amplifier 2. Next, the amplified optical noise componentsare amplified a second time in the optical amplifier 2, and then sent tothe output 8.

Since the optical noise components are filtered in the filter 4 beforegoing into the optical amplifier 2 for an additional amplification, theoptical amplifier 2 does not amplify for a second time the optical noisecomponents that are not within the predetermined optical bandwidth.

At the output, these filtered twice-amplified optical noise componentsdominate over the once-amplified optical noise components created byspontaneous emission that go directly towards the output 8.

FIG. 3 shows a schematic view of a four-pass noise source of the presentinvention. This apparatus includes an optical amplifier 20 comprised ofa doped amplifying fiber 22, such as Erbium doped fiber, a WDM 24 and apumping laser 25. The pumping laser 25 can be a multimode or a singlewavelength laser. In one embodiment of the present invention, thepumping laser 25 is a commercially available laser diode with awavelength of 980 nm or 1480 nm. The wavelength division multiplexer(WDM) 24 used in the preferred embodiment is commercially available fromGould Electronics of Glen Burnie, Md. and such WDM's are often used intelecommunications applications. The preferred embodiment of theinvention uses Erbium doped fiber as the amplifying fiber 22. Otherdoped amplifying fibers such as Praseodymium fiber, Neodymium fiber,Promethium fiber, and Ytterbium fiber can also be used.

Two lenses 26 and 28 are shown that collimate the optical components asthey leave the optical fiber 27 out of the ends 46 and 44.

Also shown is a reflective filter 32. This reflective filter 32 acts asboth the reflecting means to reflect the optical noise components backtoward the amplifier, and a filtering means to filter out the opticalnoise components not within the predetermined optical bandwidth and passoptical noise components within the predetermined optical bandwidth. Theoptical noise components that are within the predetermined opticalbandwidth are passed back towards the optical amplifier 20, and theoptical noise components not within the predetermined bandwidth are nolonger present in the optical path.

The optical path also includes a Faraday rotator 30. The Faraday rotator30 rotates the polarization of the optical noise components that gothrough it. A Faraday rotator in the preferred embodiment rotates thepolarization of the optical noise components by 45° for each pass. Inthe preferred embodiment, the Faraday rotator 30 consists of a piece ofFaraday-active material of the dimensions 2 mm×2 mm×300 μm (not shown)placed in a one-half inch cavity within a permanent magnet. ThisFaraday-active material may be a piece of (HoTbBi)IG from Mitsubishi GasChemical Company Inc. of Tokyo, Japan, but other types of Faraday-active material may be used.

A reflective filter 34 and a polarizing beamsplitter 36 form apolarizing means. The polarizing beamsplitter 36 of the preferredinvention is commercially available from the Melles Griot company of LosAngeles, Calif. and consists of a glass cube 20 mm×20 mm×20 mm made oftwo triangular sections connected together. The polarizing beamsplitter36 splits the unpolarized optical noise components sent from the opticalamplifier 20 into two different orthogonal polarizations. For example,in one embodiment TE polarized optical noise components are sent to theoutput 40, and TM polarized optical noise components pass through thepolarizing beamsplitter 36 to the reflective filter 34. The opticalnoise components that are within the predetermined optical noisebandwidth are passed back from the reflective filter 34 through thepolarizing beamsplitter 36 to the amplifier 20.

The preferred embodiment of the four-pass optical noise source includesa delay line 42. The two ends 44 and 46 of the optical fiber 27 thatcontains the optical amplifier and the delay line 42, would ideallycause no reflections of the optical components that leave ends 44 and46. In a real apparatus, however, some of the optical noise componentsthat exit the optical fiber 27 are reflected back. This reflection maycause ripples in the output signal.

The delay line 42 is made up of a single mode fiber and in the preferredembodiment is 500 m long. The delay line 42 can be used if there areripples in the output signal coming from the output 40 of the noisesource to the device 49 that uses the output signal. If the device 49that uses the four-pass optical noise source has a detection bandwidthΔν_(det) that is much greater than 1/τ, where τ is the length of thedelay line 42, the ripples in the output can be averaged out. This isbecause the frequency of the ripples in the output signal is less thanthe minimal resolvable frequency of the device 49 that uses the outputsignal.

The action of the optical noise components in the four-pass opticalnoise source can be better explained using the schematic diagram of FIG.5. FIG. 5 shows a four-pass noise source similar to that in FIG. 3.However, FIG. 5 uses a transmissive filter 80 located between the lens82 and the polarizing beamsplitter 84. This transmissive filter 80 doesnot allow any optical noise components outside the predetermined opticalbandwidth to pass from the amplifier to the polarizing beamsplitter 82.Placing the filter at this position has the benefit of filtering theoptical noise components right before the signal is sent to the output.The transmissive filter 80 placed in this position will also filter theoptical noise components before the optical noise components' third andfourth amplification.

The steps that the optical components take in the four-pass opticalnoise generator are shown in the arrows and letters at the bottom ofFIG. 5.

In step A, an unpolarized optical noise is spontaneously emitted in thedoped amplifying fiber 90 of the optical amplifier, which consists ofthe doped amplifying fiber 90, the wavelength division multiplexer orWDM 92, and the pumping laser 94. The spontaneously emitted opticalnoise leaving the doped amplifying fiber is unpolarized. Thisspontaneously emitted unpolarized optical noise can either go towardsthe Faraday rotator 96 and mirror 98 or go towards the polarizingbeamsplitter 84.

If the unpolarized optical noise goes towards the Faraday rotator 96 andmirror 98, then in step B the optical noise components are amplified afirst time, sent out the optical fiber to the lens 86, passed throughthe Faraday rotator 96 and rotated 45°. Since the optical noisecomponents are unpolarized, the 45° optical rotation leavesonce-amplified optical noise components unpolarized.

In step C, the unpolarized once-amplified optical noise componentsreflect off the mirror 98 back towards the amplifier. The once-amplifiedoptical noise components, which are unpolarized, pass through theFaraday rotator and are rotated another 45°, but remain unpolarized, andgo through the lens 86 back to the optical fiber, and to the dopedamplifying fiber 90.

In step D, the optical noise components are amplified a second time tocreate twice-amplified optical noise components which are unpolarized.These components then pass through the wavelength division multiplexer92 and most of the unpolarized twice-amplified noise components,including all of the twice-amplified optical noise components that arewithin the predetermined bandwidth, pass through the WDM 92 out of theoptical fiber through the lens 82. This occurs because the WDM passesthe relevant optical noise components through to the lens 82 and sendsan optical bandwidth including the pumping frequency to the pumpinglaser. The optical bandwidth that is sent by the WDM 92 to the pumpinglaser 94 is not part of the predetermined optical bandwidth of thefour-pass noise source.

In step E, the twice-amplified unpolarized optical noise components arefiltered in the transmissive filter 80 so that only the optical noisecomponents within the predetermined optical bandwidth pass through tothe polarizing beamsplitter 84. The polarizing beamsplitter 84 polarizesthe twice-amplified unpolarized optical noise components. Thetwice-amplified optical noise components of a first polarization leavethe polarizing beamsplitter 82 out to the output 102 as shown in stepF'. The twice-amplified optical noise components of a secondpolarization pass through the polarizing beamsplitter 84 to the mirror100 in step F. In the preferred embodiment, the second polarization isorthogonal to the first polarization.

In step G, the twice-amplified optical noise components of the secondpolarization rebound back towards the amplifier.

In step H, these components are filtered again in filter 80, passthrough the lens 82 back into the optical fiber, and pass through thewavelength division multiplexer 92 to the doped amplifying fiber 90. TheWDM 92 multiplexes the components with the pumping frequency of thepumping laser 94. The pumping frequency is then absorbed by the dopedamplifying fiber 90.

In step I, the signal is amplified a third time to createthrice-amplified optical noise components of the second polarization.These components leave the optical fiber through the lens 86 to theFaraday rotator 96.

In step J, the signal is rotated 45° in the Faraday rotator 96 from thesecond polarization to a third polarization to create thrice-amplifiedoptical noise components of the third polarization. These components aresent to the mirror 98. In step K, the components are reflected backthrough the Faraday rotator 96, which rotates the thrice-amplifiedoptical noise signal components by 45° from the third polarization tothe first polarization, and then sends the components back through thelens 86 to the optical fiber.

In step L, the noise is amplified in the doped amplifying fiber 90, tocreate four-times-amplified optical noise components of the firstpolarization. This noise then goes through the wavelength divisionmultiplexer 92 and passes out through the lens 82.

In step M, the components are filtered for a last time in thetransmission filter 80. The four-times-amplified optical noisecomponents of the first polarization pass through the polarizingbeamsplitter 84 to the output 102.

The four-times-amplified optical noise components dominate over theother components such as the twice-amplified optical components whichare output in step F'. Additionally, the optical noise components thatgo towards the mirror 100 in step A instead of towards the mirror 98 asshown in step B would be at most thrice-amplified. These components areonly thrice amplified because these components go through the dopedamplifying fiber 90, are polarized, reflect off the mirror 100, comethrough the fiber again to be amplified a second time, are rotated inthe Faraday rotator 96 twice, amplified the third time, and then sent tothe output 102 through the polarizing beamsplitter 84.

If no filtering was used, the power at the output port would beapproximately (neglecting optical coupling loss):

    P.sub.out ≈n.sub.sp G.sup.4 hνΔν

where G is the single-pass optical gain typically around 25 dB, n_(sp)is a term proportional to the level of inversion in the doped amplifyingfiber, h is Planck's constant, ν is the center frequency, and Δν is theoptical bandwidth.

If filtering is used, then Δν stands for the bandwidth of the filter.The use of the filter will prevent noise from outside the opticalbandwidth from saturating the amplifier and therefore the gain withinthe bandwidth of the filter is increased. A 1 nm bandwidth is sufficientto test the frequency response of high speed photodiodes. With a 1 nmbandwidth filter it is estimated that the noise power will be of theorder of 10 mW/1 nm.

The use of four-pass gain results in efficient use of the pump powerbecause the optical noise components have four times the possibility todelete the upper energy state, thus causing increased pump absorption.Additionally, since the pump absorption is enhanced, shorter dopedamplifying fiber lengths can be used, which reduces the cost of thedoped amplifying fiber.

FIG. 4 is a schematic diagram of an alternate embodiment of a four-passnoise source. This alternate embodiment is most useful if the opticalfiber does not display a birefringence effect. FIG. 4 shows theamplifier 50 which can be constructed out of the components shown inFIG. 2; a pumping laser 12, WDM 14 and doped amplifying fiber 16. Anunpolarized optical signal is created by spontaneous emission in theamplifier 50. This signal moves to the mirror 52, which reflects it backto the amplifier 50 to create twice-amplified optical noise componentswhich are unpolarized. These components are sent to the Faraday rotator54, which then rotates the polarization of the signal. Since the signalis unpolarized, however, the components remain unpolarized after leavingthe Faraday rotator 54. The components then go to the filter 56 whichfilters out the optical noise components that are outside of thepredetermined optical bandwidth. The twice-amplified optical noisecomponents which have been filtered are then sent to the polarizingbeamsplitter 58, where the twice-amplified optical noise components aresplit into two different polarizations. The twice-amplified opticalnoise components of the first polarization are sent to the output 62.The twice-polarized optical noise components of the second polarizationare sent to the mirror 60, which then reflects the component backthrough the polarizing beamsplitter to the filter 56 and to the Faradayrotator 54. The Faraday rotator rotates the polarization of this signalto create twice-amplified optical noise components of the thirdpolarization.

In the preferred embodiment, a 45° rotation of the components'polarization is created in the Faraday rotator. The twice-amplifiedoptical noise components of the third polarization are then amplified inthe amplifier 50 to create thrice-amplified optical noise components ofthe third polarization. These components then go to the mirror 52, whichreflects them back towards the amplifier 50. In the amplifier 50,thrice-amplified optical noise components of the third polarization areamplified to create four-times-amplified optical noise components of thethird polarization. These components go through the Faraday rotator 54,which rotates them from the third polarization to the first polarizationto create four-times-amplified optical noise components of the firstpolarization. The optical signal is filtered again to remove the opticalnoise components outside the predetermined optical bandwidth, and thensent to the polarizing beamsplitter 58, which then sends thefour-times-amplified optical noise components of the first polarizationto output 62. The filtered four-times-amplified optical noise componentsof the first polarization dominate the output.

If there is a birefringence effect in the optical fiber, then it ispreferred to place the Faraday rotator next to the mirror or reflectivefilter. Looking at FIG. 5, the Faraday rotator 96 in the preferredimplementation is placed next to the mirror 98. When a 45° Faradayrotator is placed next to the reflecting means, the Faraday rotatorcompensates for birefringence changes induced on optical signals in theoptical fiber 27 shown in FIG. 3. Because of this compensation, anoptical component that enters the optical fiber during step H will beorthogonally polarized to the optical component exiting the opticalfiber during step L despite any birefringence effect in the opticalfiber.

If the Faraday rotator was placed as in FIG. 4 on the other side of theamplifier 50 and optical fiber (not shown), then the birefringenceeffect in the optical fiber, especially the birefringence effect due toany delay line, would affect the polarization of the components sent tothe polarizing beamsplitter 58.

The change in the polarization of the optical components due to thebirefringence effect or the δΘ error in the Faraday rotator away from45° needs to be small or lasing will occur in the apparatus.

Various details of the implementation and method are merely illustrativeof the invention. It will be understood that various changes in suchdetails may be within the scope of the invention, which is to be limitedonly by the appended claims.

What is claimed is:
 1. An optical noise source comprising:an opticalamplifier that produces optical noise by spontaneous emission and emitsthe optical noise through a first side; a bandpass filter adjacent thefirst side of the amplifier; reflecting means adjacent the bandpassfilter, operative to receive optical noise emitted by the amplifier andfiltered by the filter and to reflect said filtered optical noise backinto the first side of the amplifier, the amplifier operative to amplifysaid filtered optical noise and to emit the amplified filtered noisethrough a second side; and output means adjacent the second side of theamplifier for preventing any optical noise emitted through the secondside of the amplifier from returning to the amplifier.
 2. An opticalnoise source as in claim 1 wherein the reflecting means reflects thefiltered optical noise back through the filter to the first side of theamplifier.
 3. An optical noise source as in claim 1 wherein the outputmeans comprises an optical path that transmits but does not reflect thefiltered optical energy.
 4. An optical noise source as in claim 1wherein the filter comprises a transmissive tunable filter.
 5. Anoptical noise source comprising:an optical amplifier that producesoptical noise by spontaneous emission and emits the optical noisethrough a first side; optical bandpass means adjacent the first side ofthe amplifier, operative to return substantially all of the opticalnoise that is within a predefined bandwidth back to the first side ofthe amplifier and to return substantially none of the emitted opticalnoise that is not within said predefined bandwidth back to theamplifier, the amplifier operative to amplify the returned optical noiseand to emit the amplified noise through a second side; and output meansadjacent the second side of the amplifier for preventing any opticalnoise emitted through the second side of the amplifier from returning tothe amplifier.
 6. An optical noise source as in claim 5 wherein theoptical bandpass means comprises a reflector that reflects opticalenergy within the predefined bandwidth and absorbs other optical energy.7. An optical noise source as in claim 5 wherein the optical bandpassmeans comprises a reflector that reflects optical energy within thepredefined bandwidth and transmits other optical energy.
 8. An opticalnoise source as in claim 5 wherein the output means comprises an opticalpath that transmits but does not reflect optical noise emitted by theamplifier.
 9. An apparatus or creating optical noise of a predeterminedbandwidth comprising:an amplifying means having first and second sidesin an optical path for amplifying optical noise components, saidamplifying means including means for producing unpolarized optical noiseby spontaneous emission and emitting the optical noise through the firstside; reflecting means in the optical path on the first side of theamplifying means for reflecting the optical noise produced by theamplifying means back to the amplifying means for one additionalamplification; filter means for filtering out optical noise componentsoutside the predetermined optical bandwidth and for passing opticalnoise components within the predetermined optical bandwidth, located insaid optical path between the amplifying means and the reflecting meanssuch that optical noise components passed by the filter means andreflected by the reflecting means are amplified by the amplifying meansduring said one additional amplification; and a non-reflecting output insaid optical path on the second side of the amplifying means foroutputting said twice amplified optical noise and for preventingfeedback of optical signals emitted through the second side of theamplifying means so that the twice amplified optical noise does notreturn to the amplifying means and the amplifying means does not lase.